\renewcommand\chaptername{Appendix}                      % hereafter, chapters are called "Appendix"
\renewcommand\thechapter{\Alph{chapter}}        % chapter number in Romans
\renewcommand\thesection{\Alph{chapter}.\alph{section}}  % make sections "I.a", instead of "1.1"
\setcounter{chapter}{0}                                  % start numbering chapters from 1 on again
